To many, thistle is considered an invasive weed. However, not all species of thistle are noxious weeds. Identifying which thistle is growing in your field or yard is the first step to taking control measures.
Bull thistle, Cirsium vulgare, has long leaves tipped with spines. Hair grows on both sides of the leaves and cobweb-like growth appears at the base of the flowers.
A noxious weed, this perennial thistle, Cirsium arvense, grows by seed and runners. The tops of the leaves are smooth and the small, purple-pink flowers grow in clusters.
The single, large pink or purple flowers distinguish the musk thistle, Carduus nutans. Glossy leaves have hairs that grow along the main veins. This thistle is also considered a noxious weed.
You can identify the Scotch thistle, Onopordum acanthium, by its hairy, greyish blue foliage. Another noxious thistle, it has large, spiny leaves and flowers, which turn reddish-purple.
This thistle, Cirsium altissimum, has pointed leaves with spines growing along the leaf margins. The tops of the leaves are smooth while the bottoms appear white and coated with dense hairs. The flowers are purple to pink.